The compound S-2-(3-aminopropylamino)ethyl dihydrogen phosphorothioate (which is also known as amifostine, ethiofos, Ethyol®, NSC 296961, and WR-2721 and which will hereinafter be referred to as “amifostine”) and other aminothiol compounds are disclosed in U.S. Pat. No. 3,892,824. These compounds were originally developed as antiradiation agents (radio-protectants), in particular to be used prior to exposure to x-ray or nuclear radiation, to protect against the harmful effects of such exposure which may be encountered during military conflicts.
In addition to its utility as a military antiradiation agent, amifostine has demonstrated excellent utility as a non-military radioprotectant and chemoprotectant, i.e. as a protectant administered prior to therapy to reduce the undesirable adverse effects which arise during the use of chemotherapy and radiation therapy in the treatment of cancer. Nygaard et al., eds, Radioprotectors and Anticarcinogens, Academic Press, Inc., New York, pp. 73-85 (1983); Grdina et al., Carcinogenesis (London) 6:929-931 (1985). In addition, these compounds have been reported to afford protection against the adverse effects of chemotherapeutic agents, for example, alkylating agents such as cisplatin, when administered before or concurrently with the chemotherapeutic agent. Jordan et al., Exp. Mol. Pathol. 36:297 (1982); Doz et al., Cancer Chemother. Pharmacol. 28:308 (1991). Similarly, it has been reported that amifostine has been used experimentally prior to therapy to protect HIV-infected patients (AIDS) from the harmful side effects of 3′-azido-3′-deoxythymidine (AZT) therapy. International Published Application WO 90/14007, published Nov. 29, 1990. Amifostine and its derivatives have been shown to exert these reported protective effects without affecting the beneficial properties of the administered therapeutic agents. This is, in the case of chemotherapy, believed to be due to the selective uptake of the protective thiol and other metabolites into normal tissue. Yuhas, Cancer Res. 40:1519-1524 (1980); Yuhas, Cancer Treat. Rep. 63:971-976 (1979).
Amifostine and related aminothiol compounds have also been shown to stimulate bone marrow growth. See, e g., International Published Application WO 96/25045 published Aug. 22, 1996; and List et al., Proc. Am. Soc. Clin. Oncol. 15:449 [1403] [Abstract]. Currently, amifostine is in Phase II clinical trials as a bone marrow stimulant in patients suffering from myelodysplastic syndrome. Pre-exposure with aminothiol compounds is capable of causing the bone marrow function to more rapidly recover following chemotherapy. List et al., Semin. Oncol. 23 (4) Supp. 8:58-63 (1996).
Presently, amifostine is indicated to reduce the cumulative renal toxicity associated with repeated administration of cisplatin in patients with advanced ovarian or non-small cell lung cancer. Physicians' Desk Reference 52nd ed., p. 500-502 (1998).
Amifostine is considered a prodrug. Amifostine is metabolized to the cytoprotective free thiol by dephosphorylation catalyzed by alkaline phosphatase. See, e.g. Ryan, S. V., et al., J. Clin. Pharm. 36(4):365-373 (1996). Amifostine exerts protective effects without significantly affecting the beneficial properties of the administered therapeutic agents largely because of the selective uptake of the thiol into normal tissue.
In its most common use, amifostine is administered parenterally, including by bolus injection and intravenous infusion. Amifostine is also being developed for subcutaneous administration. Since these routes circumvent the protective barriers of the human body, exceptional purity of the dosage form must be achieved. Because the dosage form must be free of microorganisms and insoluble particulates, the process used in preparing it must embody Good Manufacturing Practices (“GMP”) that will produce and maintain the required quality of the product in terms of sterility and therapeutic effectiveness. Sterility is especially important in the treatment of cancer and AIDS patients, because in many instances they are already immuno-compromised and therefor highly susceptible to infections.
The amifostine bulk drug (which is distinct from the dosage form) can exist as a bulk crystalline trihydrate which is believed to be relatively temperature stable. Such a crystalline form is described by Karle et al., Acta Cryst. C44:135-138 (1988). The bulk drug, however, is not sterile, and so cannot be reconstituted into a pharmaceutical product suitable for parenteral administration to humans.
Several methods of sterilizing bulk drugs are described in Remington's Pharmaceutical Sciences, 18th ed. (1990). These include, for example, steam sterilization, wherein a drug is exposed to high pressure steam at a minimum temperature of 121° C. This and other methods that require heating, however, cannot effectively be used to sterilize bulk crystalline amifostine. This is because crystalline amifostine loses water at about 70° C. to about 75° C. The loss of water facilitates degradation by a hydrolysis reaction that forms phosphoric acid and 2-[(3-aminopropyl)amino]ethane thiol. See, e.g., Risley, J. M. and Van Etten, R. L., Biochem. Pharmacol. 35:1453-1458 (1986). Amifostine has thus been sterilized by dissolving it in an aqueous solution which is then sterilized by membrane filtration. Substantial practical problems related to the packaging of bulk, solid amifostine using the so-called “dry filling” or “powder filling” method were thus avoided. These problems include producing sterile amifostine bulk, the difficulty in the manual manipulation of powders, the need to mill the powders to acceptable particle size and flowability, difficulty in maintaining particle-free, aseptic conditions, and the difficulty in supplying the precise dosages into individual vials.
In solution, however, amifostine is again susceptible to degradation by hydrolysis. For this reason, a prior process has sterilized an amifostine solution by filtration, and then lyophilized (freeze-dried) the amifostine solution under the following conditions: about 5 mL of a solution of 100 mg/mL amifostine and 100 mg/mL mannitol placed in vial was loaded into a freeze-drier at 0° C. and then solidified at −45° C. The vials were kept at −45° C. for 2 hours, after which time the freeze drying chamber was evacuated to 100 μm Hg. The self temperature was then raised to 0° C. over 12 hours and then maintained at 0° C. for 2 hours. The shelf temperature was finally raised to 25° C., at which temperature the vials were kept for 24 hours. This method produced a sterile amorphous form of amifostine suitable for parenteral administration to a patient (which is hereinafter referred to as “Amorphous Amifostine I”). Physicians' Desk Reference. 51st ed. (1997) p.485-486. This method also allowed easy production of vials containing predetermined amounts of sterile, lyophilized amifostine.
Unfortunately, Amorphous Amifostine I was thermally unstable and hydrolyzed over time at temperatures above 0° C. For example, a typical sample of Amorphous Amifostine I kept at 25° C. for about one month formed about 6 to 7 weight percent of the degradation product 2-[(3-aminopropyl)amino]ethane thiol. This was the state of the art when the amorphous form of amifostine was approved by regulatory authorities as having a shelf life of 24 months when stored at temperatures of between about 2° C. and 8° C. See e.g. Ethyol® European Summary of Product Characteristics (1997) (“Ethyol® SmPC”). In fact, the Amorphous Amifostine I dosage form was generally packaged, shipped, and stored at temperatures below about 8° C. Further, in the United States, the amorphous form of amifostine was approved by the Food and Drug Administration as having a shelf life of 15 months when stored at temperatures of between about 2° C. and 8° C.
Temperature instability imposes upon manufacturers and users packaging, storage and shipping requirements that increase the cost of handling the drug and make its use by hospitals and clinics more difficult, and may even prevent its use in developing countries which often lack temperature controlled storage maintenance capabilities.
In recognition of the need for a dosage form of amifostine that is more thermally-stable than Amorphous Amifostine I, research has been directed at finding new stable and sterile dosage forms of amifostine. For example, Jahansouz and coworkers studied the stability of amifostine and suggested that temperature instability of Amorphous Amifostine I is related to its degree of water content. Jahansouz, H., et al. Pharm. Res. 7(9):S195 (1990) [Abstract].
Recently, instead of attempting to increase the stability of dosage forms of amorphous amifostine such as Amorphous Amifostine I, researchers focused on creating stable and sterile crystalline dosage forms. For example, it was reported that amifostine “undergoes solid state transitions from the amorphous to crystalline phases by treatment of the freeze-dried products with varying humidity.” Further, the stability of rehydrated forms of amifostine was allegedly increased by the addition of certain excipients. Zadeii, J. M., et al., Pharm. Res. 8(10):S172 (1991) [Abstract]. This abstract does not, however, describe the rehydration process in any detail, nor does it describe the degree of stabilization provided by the two excipients it discloses. In addition, the abstract provides no detail as to how and when particular excipients should be combined with amifostine to yield the alleged more stable rehydrated forms (i.e., whether the excipients were combined with amifostine prior to, during, or after its rehydration), or whether the process should differ for different excipients. Finally, a novel dosage form of sterile, thermally-stable crystalline amifostine was discovered. U.S. Pat. Nos. 5,424,471 and 5,591,731.
Presently, that sterile crystalline dosage form of amifostine is sold under the trade name Ethyol®. Physicians' Desk Reference, 52nd ed. (1998) p. 500-502. A crystal structure and preparation of a dosage form, which exhibits greater thermal stability than Amorphous Amifostine I, is described by U.S. Pat. Nos. 5,424,471 and 5,591,731. Generally, this thermally-stable crystalline dosage form is made by dissolving bulk amifostine in a solvent solution (that allows precipitation of amifostine upon cooling below 0° C.), followed by sterilization, precipitation and lyophilization of the compound. Because the crystallinity, and hence the stability, of this amifostine dosage form requires three water molecules per molecule, the lyophilization used to provide the final product must be carefully controlled to ensure the required degree of hydration. Once made, the stable crystalline dosage form may be stored at room temperature with minimal degradation: it exhibits less than 2 weight percent degradation when kept at 40° C. for 14 days. U.S. Pat. No. 5,591,731.
Although newly developed sterile crystalline dosage forms of amifostine reduce or eliminate many of the problems associated with Amorphous Amifostine I, production of such can be costly, difficult and/or dangerous. For example, the production of crystalline Ethyol® requires the use of explosive solvents which must be handled with care and strictly isolated from oxygen. Such solvents also contain impurities not typically found in water that must be removed to ensure that the dosage form is suitable for reconstitution and administration to a patient. For example, ethanol often contains acetone, methanol, H2SO4, MnO4, and other toxic residues. See, e.g., Aldrich Catalog (1998-1999), pages 746-747. The production of crystalline amifostine dosage forms further requires long lyophilization drying times in order to ensure precise amifostine hydration.
In view of the above-described and other difficulties associated with the production of sterile crystalline dosage forms of amifostine, there remains a need for a stable, sterile amorphous dosage form, as well as an efficient and inexpensive means of its production.
The invention also encompasses non-sterile and/or bulk thermally-stable amorphous amifostine. To applicants' knowledge, to date an amorphous thermally-stable amifostine has not been previously disclosed. In preferred embodiments, the invention encompasses thermally-stable dosage forms of amifostine.